1. Field of the Invention
The present invention relates to a method of producing a lithium ion-storing/releasing material mainly composed of a metal which alloys with lithium by an electrochemical reaction such as silicon or tin and a metal oxide, an electrode structure formed of the material, and an energy storage device having the electrode structure.
2. Description of the Related Arts
It has been pointed out that global warming may occur owing to a greenhouse effect because the amount of CO2 gas in the air has been recently increasing. In addition, air pollution due to, for example, CO2, NOx, and a hydrocarbon exhausted from automobiles is of serious concern. Also in view of a run-up in crude oil prices, expectations have been placed from the viewpoint of environmental protection on hybrid vehicles and electric vehicles in each of which an electric motor to be actuated with electricity stored in an energy storage device and an engine are combined. Accordingly, the development of an energy storage device such as a capacitor or a secondary battery which has both high power density and high energy density has been desired in order that the performance of a hybrid vehicle or electric vehicle may be improved and a cost for the production of the hybrid vehicle or electric vehicle may be reduced.
Further, the functions of portable instruments such as a portable phone, a book type personal computer, a video camera, a digital camera, and a personal digital assistant (PDA) have become more and more sophisticated. The development of an energy storage device such as a secondary battery which not only has a small size, a light weight and a large capacity, but also can be charged quickly, has been desired in order that the device may be able to find use in applications including power sources for actuating the instruments.
Representative examples of the above energy storage device include the so-called “lithium ion batteries”. Each of the batteries is of a rocking chair type in which lithium ions are released by a charging reaction from between the layers of a lithium intercalation compound and lithium ions are inserted in between the layers of a carbon material typified by graphite used as a negative electrode, having a laminated structure including six-membered network planes. The batteries have been in widespread use as power sources for a large number of portable instruments because of their high cell voltages and their high energy densities. In addition, investigation has been conducted on whether each of the batteries can be used as a power source for a hybrid vehicle.
However, each of the “lithium ion batteries” can theoretically intercalate only a maximum of one lithium atom per six carbon atoms because its negative electrode is formed of the carbon material. Accordingly, it is difficult to additionally increase the capacity of each of the batteries, and a new electrode material for an increase in capacity has been desired. Although the above “lithium ion batteries” have been expected to serve as power sources for hybrid vehicles and electric vehicles because of their high energy densities, each of the batteries involves the following problem: each of the batteries has so large an internal resistance as to be incapable of discharging a sufficient electrical quantity, that is, each of the batteries has so small a power density as to be unqualified for quick discharging. In view of the foregoing, the development of an energy storage device having a high power density and a high energy density has been demanded.
The inventors of the present invention have proposed Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6, Patent Document 7, and Patent Document 8 each concerning a negative electrode for a lithium secondary battery formed of a silicon or tin element for additional increases in capacities of lithium secondary batteries including the “lithium ion batteries”.
Patent Document 1 proposes a lithium secondary battery using a negative electrode obtained by forming, on a current collector made of a metal material which does not alloy with lithium, an electrode layer formed of a metal which alloys with lithium such as silicon or tin and a metal which does not alloy with lithium such as nickel or copper.
Patent Document 2 proposes a negative electrode formed of an alloy powder made of an element such as nickel or copper and an element such as tin. Patent Document 3 proposes a lithium secondary battery using a negative electrode in which an electrode material layer contains 35 wt % or more of particles having an average particle diameter of 0.5 to 60 μm, formed of silicon or tin, and has a porosity of 0.10 to 0.86 and a density of 1.00 to 6.56 g/cm3.
Patent Document 4 proposes a lithium secondary battery using a negative electrode having silicon or tin having an amorphous phase. Patent Document 5 and Patent Document 6 each propose an active material lithium secondary battery having an amorphous phase obtained by turning a material mainly formed of a metal and inert to a material except Li into a composite with a positive electrode active material or negative electrode active material. Patent Document 7 proposes a lithium secondary battery using a negative electrode formed of amorphous tin-transition metal alloy particles having non-stoichiometric composition. Patent Document 8 proposes a lithium secondary battery using a negative electrode formed of amorphous silicon-transition metal alloy particles having non-stoichiometric composition.
A lithium secondary battery using the above amorphous alloy in its negative electrode can not only realize a large capacity but also reduce the expansion of the volume of the alloy at the time of charging. Although an approach referred to as mechanical alloying involving applying mechanical energy is an effective production method involving reducing the size of the crystallite of the above alloy to improve the amorphous property of the alloy, it cannot uniformize the composition of the alloy in a microscopic range, and cannot avoid the production of silicon oxide or tin oxide because of the following reason: a material for the alloy is turned into a fine powder to have an increased surface area, so there is no choice but to remove the material by slow oxidation. In the above alloy, lithium reacts with silicon oxide or tin oxide at the time of charging to change into an inert lithium compound such as lithium oxide which cannot release lithium reversibly, and the inert lithium compound is responsible for a reduction in charge and discharge efficiency of the battery. Further, alloy particles coated with the above inert lithium compound produced by a charging reaction each have increased electrical resistance because the compound is an insulator. In addition, when each of the alloy particles is coated with the compound non-uniformly, the intensity of an electric field applied to the particles at the time of charging become non-uniform, so alloying with lithium also becomes non-uniform, and local expansion of the volume of the alloy occurs. Moreover, it cannot be said that a reaction for alloying with lithium by storage of lithium in the silicon alloy or tin alloy lattice occurs uniformly because the alloy produced by mechanical alloying originally has non-uniform alloy composition. Accordingly, the volume expansion is still present, and an increase in internal resistance of the battery caused by the repetition of charging and discharging cannot be completely suppressed. In addition, it can never be said that the rate at which lithium is turned into an alloy at the time of charging is high, so, in quick charging, at least one of the decomposition of an electrolyte solution and the precipitation of metal lithium onto the surface of the negative electrode may occur with a certain possibility depending on the design of the structure of the battery. In view of the foregoing, the development of an energy storage device has been desired which maintains a large capacity, has a high power density, and can be charged quickly.
An electric double layer capacitor which: uses active carbon having a large specific surface area in each of its negative electrode and positive electrode; and stores electricity in its electric double layer has been expected to find use in applications including power sources for hybrid vehicles because the capacitor can be charged quickly, and has a large capacity. The electric double layer capacitor has the following major advantages: the capacitor has a long lifetime, specifically, the number of repeated uses of the capacitor is about 10 to 100 times as many as that of the “lithium ion battery”, and the capacitor has a power density about five times as high as that of the battery. However, the above electric double layer capacitor has not yet been adopted as a power source for a movable body owing to the following disadvantages: the capacitor has low energy densities, specifically, the capacitor has a weight energy density about one tenth to one half as high as that of each of the “lithium ion battery”, and has a volume energy density about one fiftieth to one twentieth as high as that of the battery. In view of the foregoing, the development of an energy storage device having increased energy densities while taking advantage of good characteristics of the above electric double layer capacitor, in other words, maintaining the following advantages has been desired: the device can be charged quickly, can be repeatedly used a large number of times, and has a high power density. A proposal concerning the use of a carbon material capable of storing and releasing lithium ions and anions at the time of charging and discharging in an electrode, and a proposal concerning a hybrid type capacitor using a metal oxide material capable of storing and releasing lithium ions at the time of charging and discharging in an electrode have been made in order that the shortcomings of the above-mentioned electric double layer capacitor may be alleviated. For example, Patent Documents 9 to 21 and Non-patent Document 1 have been proposed.
Patent Document 9 proposes a battery (an energy storage device) using a polyacene-based material capable of being doped with an ion electrochemically in at least one of its negative electrode and positive electrode. Patent Document 10 proposes a capacitor using a polyacene-based aterial in each of its positive and negative electrodes and using a quaternary ammonium salt as an electrolyte. Patent Document 11 proposes a battery (an energy storage device) using a polyacene-based material carrying lithium in advance in its negative electrode.
Patent Document 12 proposes a capacitor using a carbon material which has been caused to absorb lithium in its negative electrode and active carbon in its positive electrode. Patent Document 13 proposes a capacitor using an electrode made of a carbon material containing a metal or metal compound and having micropores in each of its positive and negative electrodes. Patent Document 14 proposes an electric double layer capacitor using, in each of its negative and positive electrodes, an electrode formed of non-porous carbon having graphite-like microcrystalline carbon in which electrolyte ions are intercalated between layers together with a solvent.
Patent Document 15 proposes an energy storage device in which a composite porous material obtained by adhering a carbon material to the surface of active carbon is used in its negative electrode and active carbon is used in its positive electrode. Patent Document 16 proposes an electric double layer capacitor formed of an electrode member obtained by electrochemically activating a carbon member and having pores larger than electrolyte ions.
A proposal in which a metal oxide is used as an electrode material has also been made. Patent Document 17 proposes an electrochemical capacitor using an electrode formed of a lithium vanadium oxide and a conductive agent as its negative electrode and an electrode formed of active carbon as its positive electrode. Patent Document proposes an electric double layer capacitor using a porous conductive ceramic having a mesoporous structure in an electrode. Patent Document 19 proposes an electric double layer capacitor using an electrode obtained by coating the surface of a porous material with a conductive ceramic.
Patent Document 20 proposes a capacitor using a carbon fine powder coated with a metal oxide, metal nitride, or metal carbide as an electrode material. Patent Document 21 proposes a lithium non-aqueous electrolyte energy storage device which: uses a composite porous material obtained by adhering a carbonaceous material to the surface of active carbon in its negative electrode and an amorphous metal oxide containing at least one of Mn and V in its positive electrode; and contains a lithium salt as an electrolyte. Patent Document 22 proposes an electrode for an electrochemical element containing an octatitanate nanosheet represented by H2Ti8O17.nH2O (n=0 to 2.0) and a carbon material. Patent Document 23 proposes a rechargeable energy battery system using a material which reversibly intercalates a cation of, for electrochemical insertion/ example, an alkali metal such as Li4Ti5O12 in its negative electrode and a material which reversibly adsorbs an anion in its positive electrode. In addition, Non-patent Document 1 reports a nonaqueous battery cell produced from a negative electrode formed of Li4Ti5O12 and a positive electrode formed of active carbon.
However, each of the above proposed energy storage devices such as a capacitor has an energy density not more than one tenth as high as that of the lithium secondary battery (including the lithium ion battery), so an additional increase in energy density of the device has been desired.
In addition, Patent Document 24, Patent Document 25, Non-patent Document 2, and Non-patent Document 3 each propose a secondary battery using carbon composite particles in which SiO is heated to cause a disproportionation reaction and Si crystals nanometers in size are dispersed in SiO2 as a negative electrode material and having good charging and discharging cycle properties. However, the above electrode using silicon dispersed in a silicon oxide involves the following problem: the amount of Li that cannot be desorbed in an extraction reaction for Li (irreversible amount) is large.
Patent Document 26 proposes that a silicon compound such as silicon from which a metal has been removed, or a material obtained by adhering a ceramic to the silicon compound be used as a negative electrode material. Si—SiO2 is obtained by mixing and heating silicon and colloidal silica, and Si—Al2O3 is obtained by mixing and heating silicon and alumina sol.
Patent Document 27 proposes, as a negative electrode material for a nonaqueous electrolyte solution secondary battery, a material formed of composite particles obtained by coating the whole surfaces, or part of the surfaces, of inorganic particles (Si, Sn, or Zn) capable of absorbing and desorbing lithium ions with a ceramic (oxide, nitride, or carbide of a material selected from Si, Ti, Al, and Zr). The above composite particles have an average particle diameter of 1 μm to 50 μm, and are prepared by: adding and mixing the inorganic particles into sol as a source of the ceramic; drying the mixture; and subjecting the dried product to heat treatment.
Each of Patent Document 26 and Patent Document 27 described above involves the following problem: the oxidation of silicon is promoted in the step of adhering the ceramic to the surfaces, so the content of silicon oxide to be produced increases, and the amount of Li that reacts with silicon oxide so as to be incapable of being desorbed in an initial electrochemical insertion/extraction reaction for Li (irreversible amount) is large.
Non-patent Document 4 announces the following: when a repetitive experiment on electrochemical insertion/extraction of Li is performed by using an electrode, which is obtained by forming a silicon nanowire on a stainless substrate with gold Au as a catalyst, as a working electrode and metal lithium as a counter electrode, coulomb efficiency for the first insertion is 73%, efficiency for each of the second and subsequent insertions is 90%, and a reduction in amount of Li to be inserted/desorbed during a period from the second insertion to the tenth insertion is small. The coulombic efficiency of the extraction of Li for the first insertion of Li is low probably because silicon oxide is formed at the time of forming the silicon nanowire.
Patent Document 28 proposes a nonaqueous electrolyte secondary battery using fibrous silicon the surface of which is coated with a carbon material as a negative electrode material. However, the document discloses neither a method of obtaining fibrous silicon nor a method of preparing fibrous silicon. Moreover, the document does not disclose any specific method of coating the surface with the carbon material.
Patent Document 29 proposes a negative electrode active material formed of: metal core particles each having a carbon-based coating layer on its surface and each containing a metal capable of forming an alloy with lithium (Si, Sn, Al, Ge, Pb, Bi, Sb, and alloys of them); and metal nanowires formed integrally to the metal core particles. The document discloses that the active material is obtained by the following procedure: a metal particle powder, a polymer material, and a pore-forming substance are mixed and baked so that the polymer material carbonizes to provide the carbon-based coating layer, and the metal nanowires grow from metal particles each contacting the carbon-based coating layer, whereby the active material is obtained. However, the document does not disclose any analysis for the shape and material of each of the metal nanowires. In addition, the document does not disclose any large-capacity negative electrode material having a charged and discharged capacity in excess of 900 mAh/g.
On the other hand, a method of producing a whisker-, wire-, or needle-like nanosilicon is proposed as described below.
Patent Document 30 proposes a production method in which a metal serving as a catalyst (Au, Cu, Pt, Pd, Ni, Gd, or Mg) is heated to melt under reduced pressure in an atmosphere containing an oxygen element as an oxidation source for silicon, and a silicon gas molecule is brought into contact with the molten metal so that a whisker-like chain is formed in which silicon crystal nanospheres each coated with an SiO2 oxide film are arrayed by the network of the SiO2 oxide film.
Non-patent Document 5 announces that the whisker of crystalline silicon is formed by: mounting a gold small particle on a silicon wafer; heating the resultant to 950° C.; and introducing a mixed gas of hydrogen and silane tetrachloride into the heated product.
Non-patent Document 6 announces that an Si Powder mixed with 0.5% of Fe is irradiated with excimer laser light in a quartz tube in an Ar gas flow at 500 Torr and 50 sccm so that nanowires each using crystalline silicon in its core and amorphous silicon oxide in its surface layer, and each having a diameter of 3 to 43 nm and a length of 2 to 3 μm are formed on the inner wall of the quartz tube.
Patent Document 31 proposes a method of growing a silicon nanoneedle involving: providing a thin film of a metal (gold, silver, or copper) that forms an alloy droplet with silicon on a silicon substrate; and heating the resultant to 1,200° C. or higher in the presence of sulfur in a vacuum inclusion closed vessel to produce silicon in a vapor phase.
Patent Document 32 proposes a production method involving evaporating silicon or a silicon/germanium alloy at a temperature equal to or lower than the melting point of silicon or the alloy, specifically a temperature in excess of 1,300° C. and 1,400° C. or lower in a stream of a carrier gas (an argon gas, a hydrogen gas, or a mixed gas of them) to grow nanowires of silicon or the silicon/germanium alloy in the temperature range of 900° C. or higher and 1,300° C. or lower. The document discloses that the produced nanowires each have a diameter of 50 nm to 100 nm and a length of several millimeters.
Patent Document 33 proposes a production method involving: evaporating sintered body of a silicon powder in a stream of an inert gas; and forming a silicon nanowire on a substrate placed at a position where a temperature gradient of 10° C/cm or more is formed in the range of 1,200° C. to 900° C. on a downstream side of the stream of the inert gas.
Patent Document 34 proposes a method of producing silicon nanowires by the heat decomposition of a polysilane gas (such as a disilane gas) with a metal that forms a low-melting eutectic alloy with silicon (gold, silver, iron, or nickel) as a catalyst under reduced pressure. The formation of silicon nanowires each having a diameter of about 50 nm and each having a length of up to 4 μm has been initiated.
However, any method of producing such nanoscale silicon as described above involves problems in that a large amount of the nanoscale silicon cannot be produced at a low cost, and the content of silicon oxide inevitably increases.
Therefore, it has been desired to develop a negative electrode material capable of providing an energy storage device having high energy density close to the energy density of a lithium secondary battery, showing high initial charge and discharge efficiency, and capable of being repeatedly used a large number of times; an electrode using the negative electrode material; and an energy storage device adopting the electrode has been desired. It has been desired to develop also a method by which a large amount of the negative electrode material can be produced at a low cost.    Patent Document 1: U.S. Pat. No. 6051340    Patent Document 2: U.S. Pat. No. 5795679    Patent Document 3: U.S. Pat. No. 6432585    Patent Document 4: Japanese Patent Application Laid-Open No. H11-283627    Patent Document 5: U.S. Pat. No. 6517974    Patent Document 6: U.S. Pat. No. 6569568    Patent Document 7: Japanese Patent Application Laid-Open No. 2000-311681    Patent Document 8: International Publication W02000/17949    Patent Document 9: Japanese Patent Application Laid-Open No. 560-170163    Patent Document 10: Japanese Patent Application Laid-Open No. H02-181365    Patent Document 11: Japanese Patent Application Laid-Open No. H04-034870    Patent Document 12: Japanese Patent Application Laid-Open No. H08-107048    Patent Document 13: Japanese Patent Application Laid-Open No. 2000-340470    Patent Document 14: Japanese Patent Application Laid-Open No. 2002-25867    Patent Document 15: Japanese Patent Application Laid-Open No. 2004-079321    Patent Document 16: Japanese Patent Application Laid-Open No. 2005-086113    Patent Document 17: Japanese Patent Application Laid-Open No. 2000-268881    Patent Document 18: Japanese Patent Application Laid-Open No. 2003-109873    Patent Document 19: Japanese Patent Application Laid-Open No. 2003-224037    Patent Document 20: Japanese Patent Application Laid-Open No. 2004-103669    Patent Document 21: Japanese Patent Application Laid-Open No. 2004-178828    Patent Document 22: Japanese Patent Application Laid-Open No. 2005-108595    Patent Document 23: U.S. Pat. No. 6252762    Patent Document 24: Japanese Patent Application Laid-Open No. 2007-42393    Patent Document 25: Japanese Patent Application Laid-Open No. 2007-59213    Patent Document 26: Japanese Patent Application Laid-Open No. 2000-36323    Patent Document 27: Japanese Patent Application Laid-Open No. 2004-335334    Patent Document 28: Japanese Patent Application Laid-Open No. 2003-168426    Patent Document 29: Japanese Patent Application Laid-Open No. 2007-115687    Patent Document 30: Japanese Patent Application Laid-Open No. 2001-48699    Patent Document 31: Japanese Patent Application Laid-Open No. 2003-246700    Patent Document 32: Japanese Patent Application Laid-Open No. 2004-296750    Patent Document 33: Japanese Patent Application Laid-Open No. 2005-112701    Patent Document 34: Japanese Patent Application Laid-Open No. 2006-117475    Non-Patent Document 1: Journal of the Electrochemical Society, 148 A930-A939 (2001)    Non-Patent Document 2: Journal of the Electrochemical Society, 153 A425-A430 (2006)    Non-Patent Document 3: Journal of Power Sources, 170 456-459 (2007)    Non-Patent Document 4: Nature Nanotechnology 3, 31-35 (2008)    Non-Patent Document 5: Applied Physics Letters 4, 89-90 (1998)    Non-Patent Document 6: Applied Physics Letters 72, 1835-1837 (1998)